1. Field of the Invention
This invention relates to a control system for controlling the boost pressure in an internal combustion engine equipped with a turbocharger and more particularly to such a system wherein the boost pressure is variably controlled with reference to presence or absence of knock.
2. Description of the Prior Art
Techniques usable in turbocharger-equipped internal combustion engines for variably controlling the boost pressure in a manner that is responsive to an occurrence of a knock are well known. One example is that disclosed in Laid-Open Japanese Publication No. 60(1985)-56128. The technique disclosed in this application involves deriving, from the engine speed and load, a basic target value to which the boost pressure is to be controlled, adjusting the basic value to a lower level when knock is detected, and raising the basic value again after knock ceases so as to increase the power output of the engine.
One problem with this system is that knock tends to occur frequently because the target value for the boost pressure is set relatively high in the vicinity of the knock limit, where knock frequently occurs. Another problem is, as a result, that the drivability of the vehicle equipped with the engine is degraded because "hunting" occurs in the power output due to the fact that the boost pressure is lowered and raised every time knock occurs. These problems become particularly pronounced when the engine is supplied with a fuel of a different octane number from that which it was designed to use.